1. Field of the Invention
The present invention generally relates to a novel lighting circuit for a vehicular discharge lamp. More particularly, this invention pertains to a novel lighting circuit with a DC booster circuit for a vehicular discharge lamp, which lighting circuit detects the status of a discharge lamp to variably control the degree of limiting the current flowing in a semiconductor switch element in the DC booster circuit, thereby suppressing an increase in consumption current at the beginning of the lighting of the discharge lamp or when the source voltage drops.
2. Description of the Related Art
Compact metal halide lamps have recently been receiving greater attention as a light source that can replace an incandescent lamp. A known lighting circuit for a vehicular metal halide lamp uses a DC power source, boosts the DC input voltage by a booster circuit and then converts the boosted voltage into an AC voltage of a sine waveform or rectangular waveform by a DC/AC converter before applying the voltage to the metal halide lamp.
The lighting circuit is also equipped with a control circuit, which performs control matching of the status of the lamp to cause the flux of lamp light from promptly becoming a steady rated level, and performs stable power control in the steady state. The output signal of the control circuit is sent to the DC booster circuit to control the boosting degree.
For instance, the control circuit is designed to detect the lamp voltage or lamp current, or its equivalent signal, cause a current several times greater than that in the steady state to flow in the lamp at the beginning of the lighting when the lighting of the lamp starts from a cold state, thereby shortening the time for the lampflux to reach the steady level, perform control in accordance with the restart voltage when the lamp has been turned off and is activated again shortly thereafter, or produce a control signal to ensure stable power control in the steady state and send the control signal to a semiconductor switch element in the DC booster circuit to carry out PWM (Pulse Width Modulation) control.
This type of lighting circuit however needs some kind of current limiting means to prevent an excessive current from flowing through the semiconductor switch element in the DC booster circuit, when the DC booster circuit is affected by a variation in battery voltage as the input voltage or a variation in load to greatly change the consumption current.
To protect the semiconductor switch element in the DC booster circuit, a limiter circuit may be provided to prevent the current from exceeding a predetermined value. With the limiter level fixed, this method undesirably permits an excessive consumption current to flow through the DC booster circuit when the battery voltage falls.
It is therefore necessary to select a greater capacity for the fuse of the power source than necessary, or use a semiconductor switch element having a high current rated value.
This is because the current limit of the limiter, which is set when the battery voltage falls within a certain specified range cannot cope with a drop of the battery voltage (in which case a greater current limit is required).
FIGS. 9A-9B presents graphs schematically illustrating time-dependent changes of the lamp flux L and the consumption current Ic ("t" representing the time). FIG. 9A shows the case where the battery voltage is in the specified range, and FIG. 9B the case where the battery voltage is low.
With the battery voltage falling in the specified range, when the consumption current Ic starts increasing from the beginning of the lighting and reaches a certain level, the lamp flux L approaches the steady value after which the Ic decreases, as shown in FIG. 9A.
In the case of FIG. 9B, however, while the battery voltage drops sharply, the power supplied to the lamp does not change much. It is apparent from this graph that the consumption current Ic greatly increases from the beginning of the lighting and has a high peak, making a change in current prominent.
The case of FIG. 9B therefore increases the possibility of causing the semiconductor switch element in the DC booster circuit to produce heat or damaging the switch element.